I used places like medichecks, a private MRI, biomesight and DNA tests via 23andme amoung a few other things to do the ruling out of other conditions. My GP was utterly useless for diagnosis as were the specialists and I had to go through the NHS list of things to rule out myself. Then I came here and started reading the research papers and ultimately managed to validate positively I had some of the biomarkers and settled on my own diagnosis. The NHS missed everything of note wrong in my body and refused to do tests that would have shown issues.
I am unable to make any part of the NHS function so its the best I can do right now. Given you are in the UK that is a good chance you will need to do similar thing since the NHS is utterly shit but it really does depend on the doctors available to you. At the very least anticipate a decade or so for diagnosis.